Edge of Forever — Margaret Pearce

Jane Smith, is an unhappy, resentful eleven year old daughter of a single parent. Her escape from the unpleasantness of life is through another world. In it she is one of the powerful four Patrollers sworn to the service of the Princess in the Far Tower on a quest to choose the bravest warriors in all space and time to fight the Red Wizard.

Jane's life improves as she makes friends, is picked for the school play and the prestigious trampoline team, but the other world with its adventures is still more important to her than the one she lives in.

The warriors' battle and the defeat of the Red Wizard causes a merging of the two worlds.